Hierarchy theory (Allen and Starr 1982, O'Neill et al. 1986) is relevant for exploration of the landscape processes. The hierarchy concept is consistent with the structural (patches) and functional (ecotopes) components of a landscape (Fig. 1.2).
Heterogeneity, which is an intrinsic feature of every landscape, can be modeled with the percolation theory of fluids. Such a theory appears useful to evaluate the degree of connectivity of the landscape mosaic as perceived by a species (Gardner et al. 1987). Theoretical landscape ecologists have been interested in this approach for at least the last two decades (e.g. Pearson and Gardner 1997). The combination of neutral models with fractal geometry and Geographic Information Systems has started to clarify the complex responses of organisms and systems to landscape heterogeneity (Milne 1997).
Meta-population theory (Levins 1970) and source-sink models (Pulliam 1988, 1996) seem to be promising frameworks from which to explore, respectively, the effects of habitat fragmentation and differentiated availability of resources. These two paradigms have a common basis in the irregular distribution of habitats and resources. In particular, meta-population theory considers the distribution of populations across fragmented and isolated habitats. The colonization/extinction rate is considered a central mechanism to maintain meta-population health. Gene flow between connected subpopulations reduces the risk of inbreeding and genetic erosion (Hanski and Gilpin 1997, Hanski 1999). Empirical and experimental evidence has quickly consolidated this theory that finds a patterned landscape as the context in which it can be adequately developed (Wiens 1997).
Source-sink models analyze population survival in terms of the balance between reproduction and death within a seasonal cycle. Such models are strictly related to habitat quality and, consequently, to the patterned landscape (Pulliam 1988,
Fig. 1.2 Some relevant theories and models incorporated in the disciplinary body of landscape ecology
1996). Recently, such models have been extended conceptually to estimate the quality of the environment and nonreproductive traits of species, like migration and over-wintering (Farina, unpublished).
To link ecosystems and habitats, the ecotone paradigm seems a very promising approach. Ecotones have received a lot of attention in recent decades, and their importance for energy distribution and the movement/distribution of organisms across a mosaic has been emphasized (Farina 1995). Ecotones are emergent characteristics of mosaic heterogeneity, and exist across spatial and temporal scales. They are species-specific and represent the position in the space-time context in which a particular organism perceives changes in environmental properties.
The ecotone paradigm can describe processes as well as environmental constraints on organisms. The capacity to extend such a paradigm throughout landscape ecology emphasizes the importance of linking ecosystems to functional landscapes. Ecotones represent the border between suitable and unsuitable habitat, delimiting in addition the safe area from the unsafe area. This paradigm is useful for describing pathways between different systems, as well as the degree of hostility of a patch in a habitat matrix (Wiens et al. 1985). Ecotone principles can explain part of the environmental complexity that is realized when different ecosystems are connected, and, consequently, it seems right to measure the ascendency properties of these systems (Ulanowicz 1997). At the same time, the ecotone paradigm can be used to evaluate the complexity of animal movements and their reluctance in crossing different environmental mosaics. Behavioral ecology (Wiens 1999) and cognitive ecology (Real 1993) could contribute to this paradigm.
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